I agree with Todd Z.
Most vehicle gas gauges and Fuel Mileage trackers are very inaccurate....especially the gas guage. You can fill up the tank and drive over 100 miles before the fuel gauge moves off of Full....but when you get below a half tank of fuel, it drops like a rock....Or it can operate just the opposite way.
I have used a Ultra-Gage that connects to the OBD port under the dash and it can be calibrated to give very accurate Speedometer, Odometer, MPG, Fuel/Miles to Empty, etc.
My 2002 Toyota Highlander was way off on MPH, Mileage, and the gas gauge was not accurate.
I installed the Ultra-Gage and found out just how inaccurate the Toyota gages were.
Same problem with my 2013 Hyundai Tucson. Speedo, Fuel Gauge and Miles to Empty were way off. Miles per tank full were off by over 90 miles but after a half tank, it started to get closer the the Ultra-Gage, and by the time I got to empty, they were within about 10 miles per tank of gas. I suspect that most of the inaccuracy was caused by the fuel gauge.
...Rich